1. Field of the Invention
The present invention relates to an electrostatic destruction preventing circuit for protecting a preamplifier at an input stage of a receiving apparatus from surge voltage of high potential inputted to the receiving apparatus due to a cloud to ground discharge or a static electricity, and more particularly to an electrostatic destruction preventing circuit for reducing the influence of a noise caused by a VHF (Very High Frequency) signal in a receiving apparatus for a medium wave band.
2. Description of the Related Art
In a receiving apparatus which receives an electric wave by an antenna, there is a case that a surge voltage of high potential is inputted thereto by an external noise such as a cloud to ground discharge and a static electricity. In order to prevent this surge voltage of high potential from destroying the amplifier at the input stage, an electrostatic destruction preventing circuit is equipped in the receiving apparatus.
There is one type of electrostatic destruction preventing circuit, which has a discharging element (i.e. surge protector) connected between an input terminal and a ground of the receiving apparatus. Thus, an electric current is escaped to the ground via the discharging element when a surge voltage of high potential is applied to the input terminal from the antenna of the receiving apparatus. In this manner, the first stage amplifier, which is connected to the input terminal, is protected by the discharging element from an excess voltage inputted from the antenna. In this electrostatic destruction preventing circuit, the response to the electrostatic destruction is certain, and there is little drawback caused by providing the electrostatic destruction preventing circuit.
However, the electrostatic destruction preventing circuit of this type has such a defect that the cost is too high.
There is another type of electrostatic destruction preventing circuit, which has Zener diodes connected between the input terminal and the ground of the receiving apparatus, as disclosed in Japanese Utility Model Application Laid Open Hei. 5-68,141. The Zener diodes are connected in reverse polarities to each other. Thus, an electric current is escaped to the ground via the Zener diodes when a surge voltage of high potential is applied to the input terminal from the antenna of the receiving apparatus. In this manner, the first stage amplifier, which is connected to the input terminal, is protected by the Zener diodes from an excess voltage inputted from the antenna. This electrostatic destruction preventing circuit has such a characteristic that it is rather simply constructed, it is cheap and it is good in a response property at the time of surge absorption. Further, the distortion is not generated in the receiving signal even under the strong electric field.
However, the electrostatic destruction of this type has such a defect that a disturbance from Other frequency band is easily generated in case that this circuit is used for a MW (Medium Wave) receiving apparatus which receives the medium wave band. Consequently, the S/N ratio obtained by the receiving apparatus is degraded.